This invention relates to a method of and apparatus for controlling the optical reading of information which is recorded on a record medium and, more particularly, to such a method and apparatus wherein the focussing of a beam of radiant energy which is used to read the recorded information is adjusted to compensate for defocussing effects which may arise because of changes in the beam transmission path.
Techniques are known wherein information is recorded on a record medium in a form capable of being reproduced, or read, by a light beam. As one example, video information is recorded in concentric, circular or spiral tracks on a rotatable disc, the information being in the form of modulated pits which are detectable by transmitting a light beam, such as a laser beam, onto the circular tracks, whereby the intensity of the light beam is modulated by the pits. If this modulated light beam then is reflected from the disc, the recorded information can be recovered by detecting the intensity modulations of the reflected beam. In other types of optical record media, information, which may be digital or analog, is recorded and a light beam is transmitted through the record medium with the intensity of the beam being modulated as a function of the recorded information.
In practical embodiments of the foregoing, the density of the recorded information is relatively high. For the example wherein video information is recorded as optically-detectable characteristics, sometimes referred to as a video disc, the circular tracks are spaced closely to each other, and the track width is very narrow. This increases the amount of information which can be recorded on the disc. Because the tracks are so narrow, the impinging light beam should be controlled so that it does not drift from the track being scanned. That is, the area of the impinging beam, referred to as the beam spot, generally has a diameter that is approximately equal to the width of a track, and the beam spot should be centered on the track being scanned. In this regard, tracking control apparatus is provided to detect tracking errors, or deviations, as the beam scans each track and to use the detected tracking error to adjust the position of the impinging beam so as to effectively center the spot on the scanned track.
Also, when the information recorded on a video disc is optically read, there is the possibility that the disc may not be perfectly flat on its bed. That is, if the disc is rotated on a turntable, eccentricities in the disc, in the turntable or in the rotating drive mechanism may displace the surface of the disc. In general, an objective lens is provided to focus the light beam precisely on the video disc so as to form the beam spot of desired size. However, this movement of the disc changes the distance between the objective lens and the surface thereof, thereby defocussing the impinging light beam. This has the effect of increasing the diameter of the beam spot, and therefore changes the intensity of the beam as reflected by the disc. Consequently, the modulations of the reflected beam intensity will include erroneous indications of information.
To minimize this problem of beam defocussing, a focus servo control system has been proposed whereby changes in the impinging beam focussing are detected and used to modify the relative position of the objective lens with respect to the surface of the video disc. This focus servo control system thus tends to maintain the impinging beam in a focussed condition. This servo control operation is performed by transmitting an additional light beam through the same objective lens as is used to focus the main beam and then to detect changes in the focus condition of the impinging additional beam. More particularly, the intensity of the additional beam, as reflected by the disc, is detected, and changes in this additional beam intensity are indicative of changes in the focus condition thereof. Since the same objective lens is used both for the additional beam and for the main beam, and since changes in the focus condition of the additional beam may be considered to be attributed to changes in the distance between the objective lens and the surface of the record disc, changes in the additional beam intensity are used to correspondingly change the position of the objective lens in a direction to maintain a substantially constant distance between that lens and the disc.
In order to avoid interference between the main light beam and the additional light beam, the additional beam is transmitted to a location on the objective lens which is spaced apart from the optical axis of the lens, the latter being the desired location along which the main beam is transmitted. Hence, the main and additional light beams are incident on the disc at spaced apart locations at its surface. However, the aforementioned tracking control technique which is used to control the proper tracking of the disc by the main beam also affects the location at which the additional beam impinges upon the disc. In general, the tracking control apparatus serves to simultaneously displace the transmission paths traversed by both the main beam and the additional beam. This means that the main beam may impinge upon the objective lens at a location spaced from the lens axis, and the additional beam likewise will impinge upon the objective lens at a still further spaced location. Now, if the focal surface of the objective lens is arcuate, then a displacement or shift in the locations at which the main and additional beams impinge upon the lens will result in respectively different focus conditions of these beams. In the usual focus servo control system, the objective lens will be moved in a direction to return the additional beam to its predetermined, focussed condition. However, because of the arcuate focal surface of the objective lens, the desired focus condition of the additional beam, which additional beam had been shifted by the tracking control apparatus, will be accompanied by a defocussed condition of the main beam which also had been shifted by the tracking control apparatus. Thus, although the usual focus servo control system operates satisfactorily in the absence of any tracking error, this servo control system provides an undesired, defocussed incident main beam in the event that a tracking error obtains.
In optical video disc playback devices, a time-base error may be present in the reproduced video signals. That is, if the rotational speed of the record disc deviates from a proper speed, this deviation results in a time-base error in the intensity modulations of the read-out beam, which time-base error appears as an error in the frequency of the reproduced chrominance component. A time-base error correcting system has been proposed for optical video disc playack devices wherein the main read-out beam is displaced in a direction along the scanning path of the beam so as to compensate for frequency changes, or time-base errors in the recovered video information. This time-base error correcting system also displaces the aforementioned additional beam and, therefore, displaces the impinging locations of the main and additional beams on the objective lens. Consequently, the focus servo control system is subjected to the same beam defocussing problem when the time-base error correcting system is operative as when the tracking error correcting system is operative.